1. Field of the Invention
This invention relates to a mold to be suitably used for injection molding of a disc such as an information recording disc.
2. Background Art
Known injection mold of the above identified type typically comprise a holder frame 2 fitted to an outer peripheral areas of the front surface of a mold fitting plate 1 and a mirror platen member 4 for a stamper 3 held in the holder frame 2 so that the stamper 3 arranged on the front surface of the mirror platen member 4 is carried by a tubular inner periphery holder 5 fitted to a central portion of the mirror platen member 4 and provided with a threaded area 5a on the outer periphery at and near the leading end thereof and an outer periphery holder 6 arranged on a stepped portion of the inside of the opening of the holder frame 2.
The inner periphery holder 5 is secured in position by placing a sprue in a central hole bored around the axis of the mirror platen member 4 from the rear side of a pilot bush 7 arranged on the front end of said central hole and introducing the inner periphery holder 5 into an annular space formed around the outer periphery of the pilot bush 7 from the front side. A locking flange 7a is integrally formed with the pilot bush 7 at the rear end thereof and the mirror platen member 4 is provided on the rear side thereof facing the locking flange 7a with a recess 9 for receiving an anchor nut 8.
Said anchor nut 8 is realized in the form of a bevel gear and capable of being rotated from the outside by means of a gear shaft 10 so that the anchor nut 8 may be brought into engagement with the threaded area 5a of the inner periphery holder 5 by rotating the gear shaft 10 to attract the inner periphery holder 5 toward the locking flange 7a until the leading end 5b of the inner periphery holder 5 abuts the locking flange 7a and the inner periphery holder 5 is placed axially in position.
With a known injection mold having the above described arrangement for anchoring the inner periphery holder, a gap is inevitably required between the anchor nut and the mirror platen member and the locking flange in order to smoothly rotate the anchor nut. Although this gap is very small, it can cause the mirror platen member to be deformed at the central portion under the pressure of resin injected for molding to produce a molded product that is slightly thicker at a central portion thereof than at the remaining portions. Particularly in the case of a product having a small diameter, the disadvantage of such a differentiated thickness becomes unnegligble because the central portion takes a significant ratio in the overall surface area.
Additionally, for the initial operation of assembling the mold and for repairing the surface of the mirror platen member, a single inner periphery holder is often commonly used for a number of molds. If such is the case, the clearance between the surface of the mirror platen member and the stamper holding edge of the tail end of the inner periphery holder has to be regulated each time the latter is applied to a new mold. The operation of regulating the clearance is typically carried out by scraping the surface of the flange for receiving the leading end 5b of the threaded area 5a of the inner periphery holder if the clearance is too large because the axial length of the inner periphery holder is to remain unchanged. Consequently, the clearance between the anchor nut and the mirror platen member is altered and hence requires a further regulating operation.
In view of the above problems, it is therefore the object of the invention to provide an injection mold that can prevent any deformation of the mirror platen member due to the gap between the anchor nut and the mirror platen member and the locking flange from taking place by securing the inner periphery holder to the mold fitting plate and allows easy regulation of the above identified clearance.